1. Introduction
This invention relates to alteration of a conductive surface utilizing a process involving electrophoretic deposition of an organic coating in an image pattern. More particularly, the invention relates to a process for forming an organic coating on a conductive substrate having discrete areas defining an image where the discrete areas have chemical compositions, resistance to attack by aggressive chemicals and solubilities in solvent or strippers differing from each other and where at least one of said discrete areas of the organic coating is applied to the conductive surface by electrophoretic deposition. In one embodiment of the invention, the organic coating is a single photoresist layer comprising distinct photoresist formulations, each in an image pattern where each image pattern is the negative image of the other and where each is developable or removable in a developer or stripper that is not a developer or stripper for the other. In another embodiment of the invention, the organic coating is a single layer comprising a photoresist and a non-imageable resist, each in a negative image pattern of the other, where the photoresist and the non-imageable resist areas are developable or removable in a developer or stripper that is not a developer or stripper for the other. A preferred embodiment of the invention is directed to the formation of printed circuit boards utilizing photoresists and electrophoretically deposited organic coatings as a single coating layer where the electrophoretic coating is substituted for an inorganic coating in the processing sequence.
2. Description of the Prior Art
The invention described below includes a process especially useful for the formation of printed circuit boards though the process is not limited to circuit board formation. Accordingly, portions of the description which follows emphasize circuit board fabrication though other uses for the process will be apparent to those skilled in the art.
The subtractive method for the manufacture of a double sided, through hole printed circuit board begins with a copper clad dielectric circuit board substrate such as a copper clad epoxy substrate. In one process, through holes are drilled through the substrate and the walls of the holes are metalized by a process involving catalysis and electroless metal deposition. Methods for metalizing through holes are disclosed in U.S. Pat. Nos. 3,011,920 and 4,585,502 incorporated herein by reference.
Following metallization of the through holes, a circuit image is formed by coating the copper cladding with an organic coating material in an image pattern. The organic coating material may be either a screen resist or a photoresist in an image pattern. Using a screen resist, the image is formed during the coating process such as by silk screening. Using the photoresist, the coating is applied as a single unitary coating by dipping, spin coating, as a dry film, by electrophoretic deposition, etc. The photoresist film is then exposed to activating radiation in a desired circuit image and the exposed coating is developed with a liquid developer capable of differentiating light exposed areas from areas that have not been light exposed and dissolving one or the other dependent upon whether the photoresist is a positive or negative photoresist. Examples of suitable photoresist materials are disclosed in U.S. Pat. Nos. 4,093,461; 4,148,654; and 4,339,516 incorporated herein by reference.
Development of a photoresist bares the underlying copper cladding on the surface of the substrate. The cladding together with the metalized through holes may then be reinforced by electroplating copper thereon. The photoresist used in the process must be able to withstand attack by the electroplating bath. This copper comprises the circuit pattern of the finished board. The remaining copper must then be removed to form the circuit. This may be accomplished by electroplating a dissimilar metal over the copper such as solder, immersion tin, gold or a tin nickel alloy. The organic coating in an image pattern permits deposition of the etch resistant metal in an image pattern. The organic coating is then removed with a solvent to bare the remainder of the copper cladding. Such solvents are known in the art and may include aqueous formulations dependent upon the photoresist used to define the image pattern. The copper cladding is dissolved by contact with a etchant which dissolves exposed copper but does not aggressively attack the etch resistant metal and therefore does not attack the copper protected by the etch resistant metal. Consequently, the etching step permanently alters the surface of the substrate--i.e., by removing exposed copper by etching.
A final step in the fabrication of a board may involve stripping (dissolving) the etch resistant metal from the board leaving the desired circuit pattern. A processing sequence such as that discussed above is set forth in Coombs, Printed Circuit Handbook, McGraw Hill Book Company, New York, 1979, chapters 6 and 7, incorporated herein by reference.
The subtractive process for making photoresists is the process most often used in the industry. However, the process is not without drawbacks. For example, to permanently alter the copper cladding--i.e., to form the circuit by etching, it is necessary to use an etch resistant metal such as solder. The application of an etch resistant metal typically involves the use of electroplating equipment including a treatment tank equipped with electroplating electrodes. Further, the electroplating process is time consuming. In addition, the need to remove etch resistant metals involves etchants to dissolve such metals. It is known that the waste treatment of etchant solutions loaded with dissolved metals is difficult and costly.